The invention relates to an electric lamp provided with a glass lamp envelope which is sealed in a vacuum-tight manner and in which a light source is arranged, the lamp envelope being coated on its inner surface with an electrostatically applied light-scattering pigmented powder layer comprising at least one cadmium compound. An incandescent lamp of this kind is known from U.S. Pat. No. 3,320,460.
Such a lamp can be used in surroundings in which it has to be avoided that the light attracts insects, such as light sources for festive illumination, disco illumination, and the like.
Electrostatically applied powder layers have the advantage with respect to layers formed from a powder suspension that during the application no solvents and binders are introduced into the lamp envelope. In fact, the powder is dusted in a dry state in a lamp envelope whose wall is given a positive potential with respect to the powder. The powder adheres to the wall under the influence thereof.
An electrostatically applied powder layer has characteristic properties which distinguish the layer from a layer formed from a powder suspension. The layer has a very small packing density, which is even fifty times smaller than the packing density of a layer formed from a suspension of the same powder mixture. The layer has at its surface a very high degree of roughness as compared with a smooth surface of a layer obtained from a suspension. A remarkable difference is further that, when an electrostatically coated lamp envelope is observed along a tangent line of the lamp envelope, it is clearly visible that the wall of the lamp envelope has a certain thickness. On the contrary, with a lamp envelope coated by means of a suspension, the wall thickness of the lamp envelope, observed in the same manner, is not perceptible.
The requirement is imposed on colored lamps that in operation and out of operation they have the same color; that is, the color is the same for transmitted and incident light, respectively. The powder layer should scatter the light produced by the lamp in such a manner that the light source is not visible and the wall of the lamp envelope is illuminated uniformly.
It has been found that, especially when deep colors are desired, for several colors, such as red, yellow and colors formed therewith, such as orange, cadmium compounds have to be used as pigments. These compounds have a great coloring power, as a result of which they color the lamp intensely despite their being mixed with the light-scattering powder. Cadmium compounds moreover have a high thermal stability. However, cadmium compounds have the disadvantage of being toxic, which is the reason why it has to be avoided that at the end of the life of the lamps large quantities of these compounds ultimately show up in the environment.